Terahertz spectroscopy

ABSTRACT

A terahertz spectroscopy system comprises a tenahertz source for illuminating, in use, a sample with a pulse of radiation in the terahertz frequency range. Excitation means provides excitation energy in the form of an electromagnetic or acoustic wave during illumination of the sample by the terahertz source and a terahertz sensor receives energy from the illuminated sample. Processing means receives signals from the terahertz sensor and processes them to provide an output representative of the terahertz spectrum received by the sensor.

Over the past few years, there has been much interest in developing thetechnique of difference spectroscopy for the investigation ofphoto-biological systems. Examples of such systems includebacteriorhodopsin and rhodopsin, as well as the study of photosyntheticreaction centers in bacteria and plants. The principle behind differencespectroscopy consists of recording infrared spectra (of, for example, aprotein) in two different states, before and after applying an externalperturbation such as light. The difference is then calculated, and onlyvibrational modes that change in intensity or frequency are identifiedin the difference spectrum. Signals that do not originate from groupsaffected by the perturbation are subtracted out. This technique isparticularly effective in probing minute structural differences betweentwo states.

Light-induced difference spectroscopy (based on Fourier transforminfrared (FTIR) technology) has been developed and applied successfullyto investigate the structural changes associated with individual bondsof chromophores and proteins. The detection sensitivity (defined asΔT/T, where T is the transmission coefficient) achieved in themid-infrared frequency range is 10⁻⁵-10⁻⁶, at fixed delay and frequency,although greater sensitivity is required to apply differencespectroscopy to other important photo-biological systems. In thefar-infrared (terahertz) range, however, the poor performance of FTIRspectrometers, owing to the lack of suitable sources and detectors,makes it impractical to develop an FTIR difference spectroscopy system.

Recent advances in terahertz (THz) time-domain spectroscopy have,though, stimulated interest in developing light-induced THz differencespectroscopy. The general THz frequency range is interpreted as thatlying between 25 GHz to 100 THz. Key benefits of such spectroscopyinclude the acquisition of time-resolved data and coherent detection.These give the amplitude and phase of the THz field, rather than simplythe laser intensity. The dynamic range of coherent THz detection hasbeen reported to be 10⁵-10⁶, corresponding to an intensity range of10¹⁰-10¹². Such a high dynamic range together with the intrinsicadvantage of time resolved coherent detection make the THz time-domainsystem attractive for differential spectroscopy.

There has also been proposed differential THz time-domain spectroscopyfor the characterization of thin dielectric films. In this approach, amechanical shaker is used to exchange rapidly the sample of interest anda reference sample at a frequency of 20-100 Hz. The small differencebetween the THz pulses transmitted through the sample and reference wasmonitored with a lock-in amplifier. Extremely high sensitivity, of theorder ΔT/T≈10⁻⁹, is apparently possible but therequirement for amechanical shaker limits the practicality of the device and makes itdifficult to control and potentially unreliable.

According to the present invention there is provided a terahertzspectroscopy system comprising:

a terahertt source for illuminating, in use, a sample with a pulse ofradiation in the terahertz frequency range;

excitation means for providing excitation energy in the form of anelectromagnetic or acoustic wave or altemative energy beam on a selectedportion of the illuminated sample prior to or during illumination of thesample by the terahertz source;

a terahertz sensor for receiving energy from the illuminated sample; andprocessing means for receiving signals from the terahertz sensor andprocessing them to provide an output representative of the terahertzspectrum received by the sensor.

The present invention provides a highly accurate device with highresolution by provision of concentrated excitation that can select asmall portion of an illuminated sample. In combination with terahertzdetection this allows for differential detection for accuratemeasurement. It also means that complex focussing of the terahertzsource to increase resolution is not necessary. The terahertz detectionmay be electro-optical or photoconductive.

The excitation means may be a laser and may be a low power laser. Thelaser may also provide the terahertz source.

Optical components may be provided in the system in order to focus theterahertz radiation onto the sample and also onto the terahertz sensor.

Means may be provided for controlling the direction of the excitingenergy to scan it across the surface of the sample in use. Correspondingmeans may be provided to control the illumination of the terahertzradiation in order to enable scanning of this also across the sample.

There may also be provided means for focussing or localising theexciting energy in order to control its spatial resolution and hencecontrol the overall spatial resolution of the system.

An example of the present invention will now be described with referenceto the accompanying drawings, in which:

FIGS. 1 a and 1 b are graphs showing the output from an example systemaccording to the present invention when measuring a semiconductorsurface and an output of an example system according to the inventionshowing peak amplitude versus time delay;

FIG. 2 is a graph showing measured terahertz signal transients for thesame semiconductor material as employed in FIG. 1 for both unexcited(open circles) and excited (lines) samples;

FIG. 3 is a diagram showing a molecular structure of a copper dyemolecule sample employed in a measurement using an example system of thepresent invention;

FIG. 4 shows two graphs indicating outputs of an example systemaccording to the present invention when measuring the sample of FIG. 3;

FIG. 5 is a plan view of a sample being illuminated by the system of thepresent invention;

FIG. 6 is a side schematic diagram showing the system of the presentinvention; and

FIG. 7 shows a plan view of a semiconductor sample and an output of thesystem according to the present invention showing clearly the increasedresolution of the system as it passes from a first to second material.

Referring now to the figures, an example system according to the presentinvention will be described, together with two example experiments usingthat system.

Referring to FIG. 6, a system 1 according to the present invention has aterahertz source 2 which is focussed through optical components 3 onto asample 4. The sample 4 is also illuminated by an exciting energy source5, which in this case is a pumped Ti:sapphire laser, but maybe othersources, such as Yb:Er doped fibre, Cr:USaf, Yb:silica, Nd:YLF, Nd:YAG,Yb, BOYS, etc. The exciting source 5 may alternatively be an acousticwave source or energy beam source, such as a neutron beam. Additionaloptional optics 6 focus terahertz radiation passing through the sample 4to a terahertz sensor 7 which provides an output to processing means 8.Alternabvely the terahertz radiation may be reflected from the sampleand detected.

As can be seen from FIG. 5, the terahertz radiation generated by theterahertz source 2, which may be excited by the lasers, can be scannedacross the surface of the sample 4 by a scanning mechanism 3 whilst thelaser beam 5 is also scanned by mechanism 10 within the confines of thespot defining the terahertz radiation, such that the whole surface ofthe sample can be evaluated in a controlled manner.

Much of the arrangement of a spectroscopy system according to theinvention is similar to that for visible-pump-THz-probe experiments. Thelaser 5 is provided and produces visible/near-infrared pulses of, inthis example, 12 fs duration at a centre wavelength of 790 nm. Theoutput is split into three parts: a 250 mW beam is used to excite sample4 with a focus diameter of 300 μm at a variable time delay with respectto the THz pulse; a 250 mW beam is focussed onto the surface of a biasedsemi-insulating Ga As (SI-GaAs) emitter for THz generation; and theremaining 25 mW serves as the probe beam for electro-optic detectionusing a 1-mm-thick ZnTe crystal. Of course the sensor 7 maybe providedby alternative crystal compositions or a photoconductor.

If photoconductive antenna detection is used, current flowing in aphotoconductor excited by a gating laser pulse is measured as a functionof delay with respect to a terahertz pulse. The optical gated pulseilluminating the photoconductor generates electron-hole pairs in a gapof the photoconductive antenna. The terahertz electric fieldco-propagating in the photoconductor drives these carriers and producesa current, its magnitude being proportional to the terahertz field.

The laser energy used to excite the samples is only a few nJ, ratherthan the few μJ used in most pump-probe experiments leading to lowenergy flux on the sample. This feature has additional benefits in thatlow energy pulses are less likely to damage the samples underinvestigation, which is of a particular concern for some biomedicalsamples.

The light-induced THz time-domain difference spectrometer system of theinvention can be operated in two ways. The first, and simplest, approachis to use the THz spectrum of the sample in its ground state (withoutlaser excitation) as the reference, and compare this with the spectrumof the sample under laser excitation. The latter can be achieved byelectrically chopping the THz beam 2 whilst maintaining constant pumplaser excitation. The difference THz spectrum is then calculated inprocessing means, and only vibrational modes that change in intensity orfrequency are detected in the difference spectrum. Signals notoriginating from groups affected by the laser excitation are subtractedout by the processing means 8.

In the second approach, if the photogenerated process underinvestigation is fast and highly reproducible, the difference THztime-domain spectrum is measured directly, with a much highersensitivity. In this case, the pump beam 5 exciting the sample ischopped by a mechanical chopper whilst the THz beam 2 is kept constant.The idea is to monitor the small THz transmission difference between thetwo sample states by alternately measuring the THz transmission throughthe excited and unexcited sample, and monitoring the difference signalwith a lock-in amplifier. Owing to the intrinsic advantage of thecoherent THz generation and detection, detection levels of the orderΔT/T≈10⁻⁸ can be demonstrated, which is already 2-3 orders of magnitudebetter than the performance of known FTIR spectroscopy systems.

Two examples of use of the present invention will now be described toaid understanding of its benefits.

Example one, semiconductor sample. In order to evaluate the performanceof the light-induced THz time-domain spectrometer of the invention,SI-GaAs and HR-silicon wafers were studied using the second approachdiscussed above. FIG. 1(a) shows the measured THz signal without thepump laser pulse (solid line) and the differential THz signal (dashedline) 5 ps after visible laser excitation for SI-GaAs. The differentialsignal has been amplified by a factor of 50 for comparison. Theamplitude of the differential THz signal is less than 2% of the originalTHz signal because only a 2 nJ pulse was used for excitation,corresponding to an energy density of about 3 μJ/cm². The peak amplitudeof the differential THz pulse was monitored as a function of time afterthe visible laser excitation and is plotted in FIG. 1(b). A 50 pslifetime was calculated by fitting the experimental results.

The differential THz signal arrives at the detector about 100 fs laterthan the original THz signal, as shown in FIG. 1(a). This can beexplained as follows. The generated THz pulse is collected and focussedonto the sample surface by two parabolic mirrors. Owing to diffractionduring the THz wave propagation, the lower frequency components of theTHz pulse will focus to a larger spot size at the sample surface thanthe higher frequency components. The visible pump laser has a spotdiameter of 300 μm and only this pumped area of the sample will producethe differential THz signal. The spatial confinement of the differentialTHz pulse in the pump area thus acts as a spectral filter, shifting thefrequency distribution of the transmitted THz pulse towards higherfrequency. This reshapes the THz waveform and, owing to the normaldispersion, slows the THz pulse down so that it arrives at a later time.The shape and peak position of the differential THz signal can be wellsimulated by applying a high-pass digital filter to the original THzsignal, confirming that the effect of a spectral filter is to cause thelater arrival of the differential THz pulse.

In contrast to our observations here, Schall et al. observed the THzpulse to arrive earlier when transmitted through an optically excitedSI-GaAs layer. This is a result of the different experimentalarrangement used. It has been known to measure the THz pulsestransmitted through an unexcited and a continuously excited GaAs layer.In this case, the frequency-dependent transmission and phase shift atthe air-GaAs (excited) interface has a substantial contribution to theobserved earlier arrival of the THz pulse. Indeed the earlier arrival ofthe THz pulse for excited HR-silicon wafers is shown in FIG. 2. The opencircles on FIG. 2 define the THz transient from the unexcited HR siliconlayer. The dashed line is the THz transient from the excited layer,which has been multiplied by a factor of 2.5 for comparison. For asilicon wafer the lifetime of the photo-generated carriers is muchlonger than the time interval between two successive optical pulses (12ns). A much larger differential THz signal is observed owing to theaccumulation of photo-generated carriers in the silicon. Consideringthat HR-silicon is widely used in THz time-domain spectroscopyexperiments, care must be taken to avoid the frequency-dependence of thetransmission and phase shift caused by long-lived photo-generatedcarriers. Note that the effective spot size of the pump beam on thesample surface in thts case is much larger owing to the diffusion of thephoto-generated electrons.

In summary, for GaAs wafers, we directly measured the differential THzsignal resulting from optical excitation. The differential signal isonly a small fraction (1-2%) of the original THz signal, therefore, thecontribution from the frequency-dependent transmission and phase shiftat the interface is much smaller for GaAs than HR-silicon. The maincontribution to the differential THz signal is thus from the spectralfilter owing to the spatial confinement of the THz pulse in the pumparea.

Example two, copper phthalocyanine pellet. Phthalocyanines are importantdye molecules with excellent light harvesting capabilities, and theirbiomedical applications have been extensively investigated. Themolecular structure of copper phthalocyanine (CuPc) is shown in FIG. 3.The optical absorption of CuPc peaks at 678 nm and overlaps with thespectrum of the pump laser pulse (centre wavelength 790 nm, bandwidth100 nm). FIG. 4 shows the THz transient transmitted through a

CuPc pellet measured in the presence and absence of visible laserexcitation. In FIG. 4(a) the solid line is the THz transient in thepresence of visible laser excitation, while the open circles representthe THz transient in the absence of visible laser excitation. Bothtransients are multiplied by a factor of ten for times greater than 10ps. The main THz pulses measured with and without laser excitation arealmost identical, suggesting that most frequency components in theabsorption spectrum do not change under laser excitation. However, thesmaller amplitude ripples after the main pulse in the time domain aredifferent, indicating that some absorption features are changed by thelaser excitation. The Fourier transforms of the measured THz transientsare calculated and the ratio of their amplitudes is plotted in FIG.4(b), togetherwith the relative phase difference. We observe features at1.08 THz and 1.26 THz. This result represents evidence of alight-induced vibrational mode change in the THz frequency range.

We do not believe that the observed change results from the mobileelectrons, which is the main cause for the differential THz signal insemiconductors. Instead, the observed peak is due to the change in theenvironment surrounding the vibrational modes. The energy associatedwith vibration modes in the THz frequency range is about 4 meV,corresponding to a temperature difference (kT) of 47° C. Therefore a fewdegrees change in temperature is sufficient to cause substantial changein either the intensity or the frequency of the THz vibrational modes.

The present invention has significant implications for THz medicalimaging. The present invention can be used in reflection mode in medicalapplications. The resolution of a THz imaging system is ultimatelylimited by the wavelength of the THz wave and although near field opticscan be used to obtain higher resolution images, this can not be appliedto in vivo THz imaging beneath, for example, the surface of skin. As isprovided by the present invention, the effective spot size of a THzpulse can be spatially confined to the pump area of a sample, which isdetermined by the focussed size of the visible pump laser beam.Therefore the resolution of a differential THz imaging system isultimately limited by the spot size of the visible pump beam rather thanthe THz wavelength. As can be seen from FIG. 7, the resolution of thedevice of the invention is significantly greater when compared to knowndevices.

1. A terahertz spectroscopy system comprising: a terahertz source forilluminating, in use, a sample with a pulse of radiation in theterahertz frequency range; excitation means for providing excitationenergy in the form of a beam on a selected portion of the illuminatedsample prior to or during illumination of the sample by the terahertzsource; a terahertz sensor for receiving energy from the illuminatedsample; and processing means for receiving signals from the terahertzsensor and processing them to provide an output representative of theterahertz spectrum Preceived by the sensor.
 2. The system of claim 1,wherein the excitation means is a laser.
 3. The system of claim 2,wherein the laser also provides the excitation for the terahertz source.4. The system of claim 1, wherein the excitation means providesexcitation energy in the form of a neutron beam.
 5. The system of claim1, wherein the excitation means provides an acoustic wave beam
 6. Thesystem of claim 1, wherein optical components are provided in the systemin order to focus the terahertz radiation onto the sample and onto theterahertz sensor.
 7. The system of claim 1, wherein means are providedfor controlling the direction of the exciting energy to scan it acrossthe surface of the sample in use.
 8. The system of claim 7, whereinmeans are provided to control the illumination of the terahertzradiation in order to enable scanning of this across the sample.
 9. Thesystem of claim 1, wherein there is provided means for focusing orlocalizing the excitation energy in order to control its spatialresolution and hence control the overall spatial resolution of thesystem.
 10. The system of claim 1, wherein the terahertz sensor is anelectro optic sensor.
 11. The system of claim 10, wherein the sensor isan EOS crystal.
 12. The system of claim 1, wherein the terahertz sensoris a photoconductive sensor.
 13. The system of claim 1, wherein theprocessing means is arranged to control the Terahertz source andexcitation means in order to control illumination of the sample.
 14. Thesystem of claim 13, wherein the processing means is arranged to controlillumination of the sample such that a reference measurement is takenwithout excitation energy on the sample and is also arranged to providea differential signal based upon a comparison between the referencemeasurement and other measurements.